1. Field of the Invention
The present invention relates to a Bloch line memory device, and more particularly to a writing system for the information carriers of a Bloch line memory device.
2. Description of the Related Art
The Bloch line memory device uses a magnetic garnet film as a memory medium film likewise to a magnetic bubble memory device. The storing systems of both the memory devices, however, differ greatly That is, in the conventional magnetic bubble memory device, the presence or absence of a bubble domain corresponds to information "1" or "0," whereas in the Bloch line memory device, the presence or absence of a vertical Bloch line pair which exists within a magnetic wall around a stripe magnetic domain generated by stretching a bubble domain corresponds to 1" or "0." FIG. 1 illustrates this situation. In the figure, upward arrows 1a and 1b in a stripe magnetic domain 1 indicate the senses of magnetization in the stripe magnetic domain 1, an arrow 101 on the center line of a magnetic wall 2 indicates the sense of the magnetization of the center line of the magnetic wall 2, and an arrow 102 vertical to the center line of the magnetic wall 2 indicates the sense of the magnetization of the center of a vertical Bloch line 3 (hereinbelow, simply called "Bloch line"). A part 4 where the Bloch lines 3 exist in a pair corresponds to the information "1," while a part 6 where they do not exist corresponds to the information "0."
The expression "Bloch line" for an information carrier signifies a fine magnetization structure which exists within the magnetic wall 2 surrounding the stripe magnetic domain 1. The Bloch line 3 can stably exist within the magnetic wall 2, and can freely move within the magnetic wall 2. Accordingly, when the stripe magnetic domains 1 in a large number are arranged in parallel at predetermined positions so as to include the Bloch lines 3 within the magnetic walls thereof, they behave just as the bubble domain which moves within the minor loop of the magnetic bubble memory device. Thus, the Bloch line memory device can take a memory device arrangement of the shift register type similarly to the magnetic bubble device.
The existence of the Bloch line has been known for long, and it has been verified by experiments and the analyses thereof that the moving speed of the magnetic domain lowers due to the existence of the Bloch line. Accordingly, in the magnetic bubble memory device wherein the magnetic domain must be moved, contrivances have been made for preventing the generation of the bubble domain containing the Bloch line, namely, the so-called "hard bubble." In contrast, in the Bloch line memory device, the existence of the Bloch line is positively utilized.
The physical size of the Bloch line is about 1/10 of the width of the stripe magnetic domain where the Bloch line exists, and a large number of Bloch lines can be caused to exist in a single stripe magnetic domain. For example, in case of a magnetic garnet film which has presently been developed for the magnetic bubble memory device and whose stripe magnetic domain has a width of 1 .mu.m, approximately 5.times.10.sup.6 Bloch lines can be caused to exist per cm.sup.2. Accordingly, in the case where the two paired lines are used as the information carrier, a memory device of 256 Mbits/cm.sup.2 -class can be fabricated.
Besides the minute size, the Bloch line has a ground permitting such a large capacity. More specifically, whereas the magnetic bubble memory propagates the information carrier by rotating an in-plane field, the Bloch line memory employs a perpendicular field for the propagation of information. Therefore, the propagation track pattern of the Bloch line memory is simple in plan, and this fact facilitates the heightened density of the device.
As stated above, the Bloch lines can store information while freely turning around the stripe magnetic domain. In order to construct the memory device, however, writing and reading information must be realized.
As regards the writing, there has been generally known a system wherein current is caused to flow through a conductor arranged at an end part of a stripe magnetic domain, to bestow a local magnetic field on the end part of the stripe magnetic domain and to invert magnetization by 180.degree., thereby to write information. It may be considered that the magnetized state indicated by "0" in FIG. 1 is inverted into the state of the region "1." On this occasion, the magnetization changes continuously at the boundary between the inverted region and the noninverted region, so that a state changing by 90.degree. with respect to the magnetic wall can be established. This corresponds to the Bloch lines. Incidentally, since this state is necessarily established with the two Bloch lines forming a pair, the information is caused to correspond to the presence or absence of one pair of Bloch lines.
Information is read out after the presence of the Bloch lines is converted into the presence or absence of a bubble domain. The conversion from the Bloch lines into the bubble domain is resorted to a method which is reported by KONISHI in "IEEE Trans., MAG-19," No. 5 (1983), pp. 1838-1843. This method will be explained with reference to FIGS. 2A and 2B. When Bloch lines 3 exist within the magnetic all 2 of a stripe magnetic domain 1, the sense of magnetization within the magnetic wall 2 is inverted with the boundary at the Bloch lines 3. Owing to such a change in the structure of the domain wall, easiness in the chopping of the end part of the magnetic domain 1 becomes different between in a case where one Bloch line 3 has moved to the end part of the stripe magnetic domain 1 as illustrated in FIG. 2A and in a case where no Bloch line exists at the end part of the stripe magnetic domain 1 as illustrated in FIG. 2B. Thus, only in the case where the single Bloch line 3 exists at the end part of the stripe magnetic domain 1 as in FIG. 2A, a predetermined current is caused to flow through a chopping conductor 7 disposed in the vicinity of the end part of the stripe magnetic domain 1, whereby a bubble domain 8 can be chopped from the end part of the stripe magnetic domain 1. On the other hand, in the case where the Bloch line 3 does not exist at the end part of the stripe magnetic domain 1 as in FIG. 2B, the bubble domain cannot be chopped even when the current is caused to flow through the chopping conductor 7. The bubble domain 8 chopped in FIG. 2A is propagated by an expedient similar to the major line of the bubble memory device, and is converted into an electric signal. Then, the presence of the Bloch line can be read out.
By fabricating portions for the above writing, storing and reading functions on an identical device, the Bloch line memory device can be realized.
The writing and reading stated above are described in U.S. Pat. No. 4,583,200.
A magnetic storing method wherein the magnetized state of a stripe magnetic domain is set at S=0 (the state in which two Bloch lines exist), is also described in U.S. Pat. No. 4,583,200. Now, this method will be briefly explained with reference to FIGS. 3A-3D.
FIG. 3A illustrates a state in which two pairs of Bloch lines 8a and 8b exist within a magnetic wall 2 around a single stripe magnetic domain 1. The senses 9a and 9b of magnetization are opposite to each other between the Bloch line pairs 8a and 8b. The reason is that, as seen from FIG. 1, the magnetization rotates continuously along the magnetic wall. Under this state, a magnetic field H in the same direction as the sense of magnetization 9b in the Bloch line pair 8b in FIG. 3A is applied. Then, the pair of Bloch lines 8b are separated to move to both the ends of the stripe magnetic doamin 1 Consequently, information possessed by the Bloch line pair 8b disappears (FIG. 3B). As an expedient preventive of this drawback, there has been considered a measure of improvement in which one Bloch line 31 or 32 is inserted at each end of the stripe magnetic domain 1 (FIG. 3C). When an in-plane field H.sub.ip is applied with the Bloch lines 31 and 32 existing at both the ends of the stripe magnetic domain 1 as shown in FIG. 3C, the senses of the magnetization of the stripe magnetic domain 1 become the same as the direction of the in-plane field H.sub.ip on both the upper and lower sides in the drawing, and hence, the Bloch lines 31 and 32 can stably exist at the end parts. When, under this state, the Bloch line pairs 8a and 8b are written, the senses of magnetization 9a and 9b between the Bloch line pairs are reversed to the in-plane field H.sub.ip by the continuity of the direction of the magnetization stated above (FIG. 3D). Therefore, whether the Bloch line pair exists on the upper side or the lower side of the magnetic wall 2, it can stably exist without separating.
Next, a prior-art system for writing Bloch lines will be described. FIG. 4A is a diagram showing the right-side part of FIG. 3C. In the illustration of FIG. 4A, a bias field H.sub.B is applied perpendicularly to the sheet of the drawing. Besides, a Bloch line 31 is caused to exist at the end part of a stripe magnetic domain 1 in order that the magnetization of a magnetic wall 2 may entirely become identical in direction to an applied in-plane field H.sub.ip.
In performing a writing operation, first of all, the Bloch line 31 is moved counterclockwise as shown in FIG. 4B. Subsequently, a pulse current I.sub.p is caused to flow through a conductor 10 so as to form a Bloch line pair 81. The Bloch line pair 81 consists of Bloch lines 33 and 34. The magnetization directions of the Bloch line pair 81 infallibly become the same owing to the continuity of magnetization. The Bloch lines of equal magnetization directions are annihilated when combined. Under this condition, therefore, they do not form an information carrier.
In order to solve this drawback, the previously existing Bloch line 31 is paired with the Bloch line 33 as indicated at symbol 8a in FIG. 4C, and the pair 8a is used as the information carrier. In this case, it is an indispensable requirement that the directions of the magnetizations of the previously existing Bloch line 31 and the Bloch line 33 are opposite to each other. This requirement may be met in such a way that the polarity of the pulse current I.sub.p to flow through the conductor 10 is properly selected so as to control the directions of magnetization of the Bloch line pair 81.
Meanwhile, under the state of FIG. 4C, the Bloch line 34 the magnetization direction of which differs by 180.degree. from that of the Bloch line 31 in the initial state is left behind at the end part of the stripe magnetic domain 1. In order to change the Bloch line 34 into the Bloch line 31 in the initial state, as illustrated in FIG. 4D, the end part of the stripe magnetic domain 1 needs to be chopped as a magnetic domain 12 by causing current I.sub.p ' to flow through a hairpin conductor 11. Owing to this chopping operation, a Bloch line 35 having the same magnetization direction as in the initial state is formed at the end part of the stripe magnetic domain 1 nearer to the Bloch line pair 8a written for the information carrier.
As described above, the writing system of the prior art Bloch line memory is materialized broadly by the three steps (FIGS. 4B, 4C and 4D).
That is, the first step is the movement of the Bloch line 31 at the front end, the second step is the formation of the Bloch line pair 81, and the third step is the chopping of the magnetic domain 12 for replacing the Bloch line 34 at the front end with the Bloch line 35 in the initial state. Since, in this manner, the prior art has required the complicated operation for the writing of information, it has been difficult to drive the memory device at high speed.